EP-A-233466 discloses a system and a process for the biological purification of waste water, wherein BOD decomposition, nitrification and denitrification are carried out simultaneously in a reactor in which there are microorganisms attached to a carrier. The carrier type is such that the biomass can move freely through the waste water. The purified water and the sludge are then separated in a secondary settling tank, whereafter the sludge is returned to the reactor.
EP-A-24758 discloses a process and a system for oxidative biological purification of waste water, the waste water being passed upwards through an oxidation chamber in which there are microorganisms attached to an insoluble carrier. Separation of the purified waste water and the active sludge takes place at the top of the oxidation chamber, all of the sludge being returned to the oxidation chamber. In this case, both COD purification and nitrification take place.
EP-A-28846 discloses a process for preparing biomass attached to a carrier, the carrier, for example sand, being brought into contact, while being aerated using a sufficient amount of mechanical energy, with a liquid which contains the required microorganisms and nutrients.
A process and an apparatus for aerobic treatment of waste water are known from EP-A-268225. The apparatus is divided in two reaction chambers, one being gas-rich and the other being gas-depleted, and is provided with a settling chamber.
EP-A-399380 discloses an apparatus suitable for denitrification and nitrification of waste water comprising a reactor having an internal vertical division allowing a circulation of the waste water with filling material carrying the biomass. A denitrifying zone is provided in the funnel-shaped bottom part of the reactor.